Blast furnace

A blast furnace is a type of furnace for smelting whereby the combustion material and ore are supplied with air from the bottom of the chamber such that the chemical reaction does not take place only at the surface. Typically this is used for the production of pig iron from iron ore.

History

The oldest known blast furnaces were built in Han China in the 1st century BC. However, cast iron artifacts found in China have been dated as early as the 5th century BC, so it is possible that the history of the blast furnace in China is older than presently known. These early furnaces had clay walls and used phosphorus-containing minerals as a flux. The oldest known blast furnaces in the West were built in Sweden at Lapphyttan, sometime between 1150 and 1350. It is unclear whether the blast furnace was independently developed there, or whether the technology was transmitted to Europe by the Mongols.

In 1709, Abraham Darby developed a blast furnace that could be fired by coke instead of charcoal. Deforestation in Europe had progressed to the point that fuel-wood was becoming a strategic resource, and the use of a coal-derived fuel made blast furnace-based iron smelting significantly less expensive.

Process

The blast furnace relied on the fact that the unwanted sulfur-iron compounds were lighter than the pure iron and iron-carbon mix, pig iron, that was its main product. The furnace was built in the form of a tall chimney-like cauldron lined with refractory brick. Coke, limestone and iron ore are poured in the top, which would normally burn only on the surface. Pre-heated air was blown into the middle, thus the "blast", allowing combustion in the middle of the mixture. The results of this localized burning was a liquid that sank to the bottom of the furnace, with the lighter materials on top. A valve was opened to allow the slag to pour out, and once emptied, another valve at the bottom opened to remove the pig iron.

The exact nature of the reaction is:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Air blown into the furnace reacts with the carbon in the coal to produce carbon monoxide, which then mixes with the iron oxide, reacting chemically to produce pure iron and carbon dioxide, which leaks out of the furnace at the top.

The temperature in the furnace typically runs at about 1500°C, which is enough to also decompose limestone (calcium carbonate) into calcium oxide and additional carbon dioxide:

CaCO₃ → CaO + CO₂

The calcium oxide reacts with various acidic impurities in the iron (notably silica), forming calcium silicate, CaSiO₃ and floats with the slag, thereby further purifying the iron.

The pig iron produced by the blast furnace is not very useful directly due to its high carbon content, around 4-5%, making it very brittle. Further processing was needed to reduce the carbon content for use as a construction material. This is done by forcing a jet of high-pressure oxygen into a special rotating container containing the pig iron. Some of the carbon is oxidised into carbon monoxide, CO, and carbon dioxide, CO₂. This also oxides impurities in the pig iron. The container is rotated and the processed pig iron can be separated from the oxidised impurities. For some time, the product of the blast furnace was used almost directly as wrought iron after additional processing, the conversion to steel using the crucible technique was too expensive to operate on a large scale. However with the introduction of the Bessemer process the conversion to steel was also dramatically improved, and by the turn of the late 1800s almost all iron was being converted to steel before use.

The blast furnace remains an important part of modern iron production. Modern furnaces include cowper stoves to pre-heat the blast air to high temperatures in order to avoid cooling (and thus having to re-heat) the mix, and use fairly complex systems to extract the heat from the hot carbon dioxide when it escapes from the top of the furnace, further improving efficiency. The largest blast furnaces produce around 60,000 tonnes of iron per week, enough for about four cars per minute.

Metallurgists use the name cupola to refer to a cylindrical shaft type of blast furnace used for remelting metals (usually iron) before casting.